Spectral diagonal ensemble Kalman filters
Summary: A new type of ensemble Kalman filter for data assimilation is developed, based on fast Fourier transform and wavelet transform. The method can work with minimal computational resources. We develop variants for several general types of observations, give a rigorous proof that the method improves the approximation of the state covariance, and present computational experiments showing that the new technique works reliably with very small ensembles and is stable over multiple analysis cycles.
Nonlin. Processes Geophys., 22, 485-497, doi:10.5194/npg-22-485-2015, 2015
Search for the 531-day-period wobble signal in the polar motion based on EEMD
Summary: A 531-day wobble (531 dW) signal is clearly detected with a mean amplitude of about 7 mas after applying the ensemble empirical mode decomposition (EEMD) to the 1962-2013 polar motion (PM) time series. This signal is also detected in the two longest available superconducting gravimeter (SG) records. Synthetic tests are carried out to explain why the 531 dW signal can only be observed in recent 30-year PM time series after using EEMD.
Nonlin. Processes Geophys., 22, 473-484, doi:10.5194/npg-22-473-2015, 2015
Direct numerical simulation of intermittent turbulence under stably stratified conditions
Summary: The present work, to the best of our knowledge, is the first DNS study reporting temporal (long-term) statistics of intermittent turbulence (a.k.a. bursting events) in stably stratified flows. The present paper not only accurately reproduces recently published key spatial statistics of intermittent turbulence but also reports some intriguing features, e.g., the coexistence of internal waves and intermittent turbulence.
Nonlin. Processes Geophys., 22, 447-471, doi:10.5194/npg-22-447-2015, 2015
Global terrestrial water storage connectivity revealed using complex climate network analyses
Summary: Terrestrial water storage (TWS) plays a key role in global water and energy cycles. This work applies complex climate networks to analyzing spatial patterns in TWS. A comparative analysis is conducted using a remotely sensed (GRACE) and a model-generated TWS data set. Our results reveal hotspots of TWS anomalies around the global land surfaces. Prospects are offered on using network connectivity as constraints to further improve current global land surface models.
Nonlin. Processes Geophys., 22, 433-446, doi:10.5194/npg-22-433-2015, 2015
The evolution of mode-2 nonlinear internal waves over the northern Heng-Chun Ridge south of Taiwan
Summary: Highly energetic mode-2 nonlinear internal waves were found to be generated at several sites near the northern Heng-Chun (western) Ridge south of Taiwan. The local environment however was highly dissipative, and the energy budget suggests that little wave energy escapes the ridge to contribute to the large transbasin waves that have been previously observed in other studies.
Nonlin. Processes Geophys., 22, 413-431, doi:10.5194/npg-22-413-2015, 2015
Verification against perturbed analyses and observations
Nonlin. Processes Geophys., 22, 403-411, doi:10.5194/npg-22-403-2015, 2015
Spatial random downscaling of rainfall signals in Andean heterogeneous terrain
Nonlin. Processes Geophys., 22, 383-402, doi:10.5194/npg-22-383-2015, 2015
Nonstationary time series prediction combined with slow feature analysis
Summary: This paper presents a new technique of combining the driving force of a time series obtained using the slow feature analysis (SFA) approach, then introducing the driving force into a predictive model to predict nonstationary time series. It could be considered to be a data-driven attempt to make progress in predicting nonstationary climatic time series and in better understanding the climate causality research from observed climate data.
Nonlin. Processes Geophys., 22, 377-382, doi:10.5194/npg-22-377-2015, 2015
Improved singular spectrum analysis for time series with missing data
Nonlin. Processes Geophys., 22, 371-376, doi:10.5194/npg-22-371-2015, 2015
Reversals in the large-scale αΩ-dynamo with memory
Nonlin. Processes Geophys., 22, 361-369, doi:10.5194/npg-22-361-2015, 2015
Stress states and moment rates of a two-asperity fault in the presence of viscoelastic relaxation
Summary: The paper presents new analytical solutions for both the coseismic slip and the interseismic evolution of a fault with two asperities of different strengths. It enlightens the relationship between the state of the fault before a seismic event and the number and sequence of slipping modes in the event. It shows that the knowledge of the source function of a seismic event constrains the subsequent evolution of the system. The model is applied to the fault that generated the 1964 Alaska earthquake.
Nonlin. Processes Geophys., 22, 349-359, doi:10.5194/npg-22-349-2015, 2015
Dynamics of turbulence under the effect of stratification and internal waves
Summary: The objective of this paper is to study the dynamics of turbulence near a pycnocline, both in the free regime and under the action of an internal wave (IW) propagating along a pycnocline by direct numerical simulation (DNS). Turbulence is initially induced in a horizontal layer above the pycnocline. The DNS results show that turbulence kinetic energy (TKE) is significantly enhanced as compared to the TKE in the absence of IW, and most of the TKE is localized in the vicinity of the pycnocline.
Nonlin. Processes Geophys., 22, 337-348, doi:10.5194/npg-22-337-2015, 2015
Evaluation of a spectral line width for the Phillips spectrum by means of numerical simulation
Nonlin. Processes Geophys., 22, 325-335, doi:10.5194/npg-22-325-2015, 2015
An analytical model of the evolution of a Stokes wave and its two Benjamin–Feir sidebands on nonuniform unidirectional current
Summary: An analytical weakly nonlinear model of Benjamin–Feir instability of a Stokes wave on nonuniform unidirectional current is presented. In contrast to the models based on versions of the cubic Schrodinger equation the current variations could be strong, which allows us to examine the blockage of waves. Waves may overpass the blocking barrier produced by strong adverse current. We find reasonable correspondence between the results of model simulations and available experimental results.
Nonlin. Processes Geophys., 22, 313-324, doi:10.5194/npg-22-313-2015, 2015
Two-dimensional numerical simulations of shoaling internal solitary waves at the ASIAEX site in the South China Sea
Summary: Two-dimensional numerical simulations of the shoaling of an internal solitary wave (ISW) in the South China Sea have been undertaken. Peak amplitudes are attained at depths of 250 and 600m. Horizontal resolutions of 50m are required to simulate the formation of a pedestal in shallow water behind the shoaling wave. At a depth of 200m, waves can exceed maximum ISW amplitudes by 50%. Sensitivity to the bathymetry and stratification and the effects of rotation are considered.
Nonlin. Processes Geophys., 22, 289-312, doi:10.5194/npg-22-289-2015, 2015
Oscillations in a simple climate–vegetation model
Summary: Our conceptual model describes global temperature and vegetation extent. We use elements from Daisyworld and classical energy balance models and add an ocean with sea ice. The model exhibits oscillatory behavior within a plausible range of parameter values. Its periodic solutions have sawtooth behavior that is characteristic of relaxation oscillations, as well as suggestive of Quaternary glaciation cycles. The model is one of the simplest of its kind to produce such oscillatory behavior.
Nonlin. Processes Geophys., 22, 275-288, doi:10.5194/npg-22-275-2015, 2015
Incidence and reflection of internal waves and wave-induced currents at a jump in buoyancy frequency
Nonlin. Processes Geophys., 22, 259-274, doi:10.5194/npg-22-259-2015, 2015
A novel method for analyzing the process of abrupt climate change
Summary: A novel method is created to detect the process of the abrupt change, which has not been mentioned yet in traditional research. By building an ideal time series with a transition process, the results show that the process could be detected clearly. When applied to a climate index, this method detects five processes, and all of them have reappeared via the “start-end states” phase diagram. Additionally, it is detectable that the persist time of the process is related to global warming.
Nonlin. Processes Geophys., 22, 249-258, doi:10.5194/npg-22-249-2015, 2015
Data assimilation experiments using diffusive back-and-forth nudging for the NEMO ocean model
Nonlin. Processes Geophys., 22, 233-248, doi:10.5194/npg-22-233-2015, 2015
Estimation of the total magnetization direction of approximately spherical bodies
Summary: We have developed a fast total-field anomaly inversion to estimate the magnetization direction of multiple sources with approximately spherical shapes and known centres. It requires neither the prior computation of any transformation-like reduction to the pole nor the use of regularly spaced data on a horizontal grid. The method contains flexibility to be implemented as a linear or non-linear inverse problem. Applications to synthetic and field data show the good performance of our method.
Nonlin. Processes Geophys., 22, 215-232, doi:10.5194/npg-22-215-2015, 2015